Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or evoked responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or evoked responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, one electrode serving as the anode and the other electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Implantable cardiac defibrillators (ICD's) are also well known in the art. These devices generally include an arrhythmia detector that detects accelerated arrhythmias, such as tachycardia or fibrillation. When such a tachyarrhythmia is detected, a pulse generator delivers electrical therapy to the patient's heart. A therapy for tachycardia may be anti-tachycardia pacing and a therapy for fibrillation may be a defibrillating shock. Such therapies for both atrial and ventricular tachyarrhythmias are well known.
With ventricular tachycardia (VT) the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, a ventricular tachycardia is typically not immediately fatal. However, ventricular fibrillation (VF) is an immediately life threatening tachyarrhythmia for which the device takes immediate and aggressive action. Such action is required because during VF, the heart beats chaotically such that there is little or no net flow of blood from the heart to the brain and other organs.
With atrial tachycardia (AT), the atria of the heart beat rapidly at an abnormally high rate. This can cause the ventricular to in turn beat at a high rate. Cardiac output is reduced. The patient may experience dizziness or feel fatigued. Although not immediately life threatening, it can also be unpleasant to a patient.
Atrial fibrillation (AF) is a common atrial tachyarrhythmia and can occur suddenly. It results in rapid and chaotic activity of the atria of the heart. The chaotic atrial activity in turn causes the ventricular activity to become rapid and chaotic. Under these conditions, patients also experience dizziness or feel fatigued due to reduced cardiac output. Normal daily activities must be suspended. Although not life threatening, AF is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition to strokes, symptoms of atrial fibrillation may include fatigue, syncope, congestive heart failure, weakness and dizziness.
Stabilizing the ventricular rhythm during episodes of AF is of great concern. A stabilized ventricular rate during episodes of AF would add greatly to the quality of life of such patients. Drugs are known which can assist some patients toward a more stable ventricular rate during episodes of AF. Unfortunately, many patients with chronic AF are resistant to such drugs. One therapy for such patients is to ablate their atrioventricular (AV) node. The AV node is a small concentration of specialized connective tissue at the base of the atrial septum which transmits signals from the atria to the ventricles. When a cardiac cycle of the heart is initialized, an electrical signal first causes the atria to be activated (contract) and then, after being transmitted by the AV node and conducted to the ventricles, causes the ventricles to be activated (contract). The time between the atrial activation and the ventricular activation is referred to as an AV interval. Ablation of the AV node thus destroys the AV nodal function of transmitting the electrical signals from the atria to the ventricles. These patients are pacemaker dependent in that they must receive and maintain a pacemaker implant to pace the ventricles. During episodes of AF, the ventricles continue to be paced at an appropriate rate independent of the rapid and chaotic atrial rate.
An alternative albeit experimental approach was to ablate only parts of the atrium to block conduction from the atrium to the AV node and to place a lead in the AV node or His Bundle. While the ventricles were captured, significant parts of the atrium needed to be ablated.
It would thus be desirable if the ventricular rate could be stabilized during episodes of AF towards preserving normal conduction patterns in the ventricles without the need for ablation The present invention addresses these and other issues.